fix(techdocs): use correct type for additionalAllowedUriProtocols

Signed-off-by: Bailey Everts <beverts312@users.noreply.github.com>

.changeset/gentle-results-lie.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-techdocs': patch
+---
+
+Fixed schema type for additionalAllowedURIProtocols

plugins/techdocs/config.d.ts

@@ -67,7 +67,7 @@ export interface Config {
        *  @see: https://raw.githubusercontent.com/cure53/DOMPurify/master/src/regexp.ts
        * @visibility frontend
        */
-      additionalAllowedURIProtocols?: string;
+      additionalAllowedURIProtocols?: string[];
     };
   };
 }